PERF: Split CSS by color theme

[designerbrent]

In reviewing Google Chrome's coverage reports, I've noted that on a given page, over 90% of the CSS is not used. I realize this is one page, but I also know that with 16 color themes, we are pushing WAY more code then we need to push on each page load.

What would it take to refactor the CSS so we could render 17 options for the main.css file, one unified file (like we have now) and one for each color theme, for example main-treefrog.css?

This would allow a refactoring of inclusion code in the Wordpress and other CMS themes to only include the CSS needed for each theme.

Thoughts? @davideleuterius @kelscahill @eduludi @sergsadovyi

[designerbrent]

@sergsadovyi Can you take a look at this issue as at the same time as #457? I think this means the version.json file will be expanded to include the URL for each version of the CSS.

[designerbrent]

This has now been implemented starting in version 3.6.3 and can be seen in the versions.json file.